Canning or otherwise packaging foods to preserve and store them for long periods of time has been an important part of food processing since the eighteenth century, when a Parisian chef named Appert devised a crude method of canning. Appert's process was introduced into the United States through England in about 1818. Canning remained an inexact process until Louis Pasteur applied his principles of fermentation to it in 1895.
Today, from picking to packaging, canning is a highly-developed, scientific industry. Foodstuffs are packaged in many different types of containers, with metal cans, glass jars and plastic packages being used on a wide scale. For convenience, the packaging of foodstuffs in rigid containers (i.e., cans, jars and rigid plastic packages) will be referred to collectively herein as "canning". No matter what type of container is used, however, all canning processes must deal with the sensitivity of most foods to oxygen. As anyone who has sliced a fresh apple knows, oxygen in the air immediately begins to react with fresh foods and leads to the loss of their organoleptic qualities and to their rapid spoilage. All foods are sensitive to oxygen in varying degrees, and the successful preservation of foods by canning requires, as an important step, the elimination of oxygen from the containers.
Conventional canning processes rely on the use of liquids to displace oxygen and other gases from the containers. Typically, the foodstuffs being canned are placed in the containers and then covered with a liquid, which may be water, brine, or syrup. The covering liquid is preheated to a temperature of about 100.degree. C. before it is added to the containers. The liquid thus displaces the air and other gases in the containers. The containers are then sealed while at that temperature. The heated liquid at temperatures near 100.degree. C. is hot enough to begin cooking the foodstuffs even before the containers are subjected to further processing, such as sterilizing. Usually, the containers are also further heated to temperatures between 115.degree. C. and 140.degree. C. in order to sterilize the contents.
Clearly, the conventional methods of canning, while offering many advantages, also suffer certain drawbacks. Raising the temperature of the food to the boiling point at least partially cooks the food, resulting in some loss of texture, color, taste and other indicia of freshness. In addition, the liquid in which the food is packaged can itself react with the food, or with the container, which can lead to undesirable changes in taste, color or aroma.
Attempts have been made to overcome these drawbacks, but they have not been entirely successful. Some foods which do not react vigorously with oxygen, such as sweet corn, can be processed in a vacuum obtained by pumping out the gases from the container before sealing it and sterilizing it. The level of vacuum so obtained is limited, however. Moreover, this method is not suitable for most of the aqueous foods (e.g., fruits and vegetables with high water content), since these foods include oxygen-sensitive compounds such as polyunsaturated fatty acids, tannins, vitamins, and so forth. Even low oxygen levels resulting from vacuum packaging are enough to lead to deterioration of aqueous foods so packaged.
A recent approach to overcoming the problems with canning aqueous foods is shown in U.S. Pat. No. 4,717,575. In that patent, aqueous foods are placed in containers such as metal cans and covered with water. The cans are filled with water to the brim, so that water completely covers the foods. The cans are then placed in a steam chamber in which they are surrounded by a steam atmosphere substantially devoid of air. While in the steam chamber, the cans are either tilted or inverted to cause some or all of the water to be removed from the cans. The water thus removed from the cans is replaced by steam from the surrounding steam atmosphere. The cans are then sealed in the steam atmosphere at either atmospheric or superatmospheric pressure. After sealing, the cans are cooled. The cooling step causes the water vapor in the cans to condense, creating a vacuum in the cans.
The process disclosed in U.S. Pat. No. 4,717,575 is cumbersome and does not solve the problems of conventional canning methods. The process requires a steam chamber with a steam atmosphere at pressures equal to or greater than atmospheric pressure. The dangers of steam pressure vessels are well-known and need not be repeated here. The process also requires the cans to be filled to the brim with boiling water before the cans are introduced into the steam chamber, but then the water is poured out inside the steam chamber to allow steam to replace it. (As a practical matter, boiling water must be used in order to exchange the water for steam. If the water were cooler than 100.degree. C., it would partially or totally condense the steam, making the substitution of steam for water impossible.) This is not only wasteful, but requires complex machinery for tilting or inverting the cans, and a perforated grille on the cans to keep the food inside the cans while the water is being poured out. What is worse, the subjection of the food in the containers to boiling water and then to high-temperature steam partially cooks the food, leading to a degradation in the food's organoleptic properties and freshness appeal. The process further requires the cans to be sealed inside the steam chamber, which necessitates sealing equipment that can operate in the severe, corrosive environment of wet steam.
The present invention provides a process which attains a high level of vacuum in the containers at low temperatures, eliminating the air and other gases from the containers by a novel and inventive combination of pressures and temperatures. The present invention makes it possible to obtain a hermetically sealed container devoid of air and oxygen without cooking the product. Moreover, no special, complex equipment is required to invert cans, or to seal cans inside a high-temperature steam environment. Only little liquid is used compared to conventional methods, which makes the process much more economical than prior processes which require filling the containers with liquid and then pouring off the liquid before sealing.
The process of the present invention embodies the advantages of canning without the concomitant disadvantages of prior processes, and results in a canned foodstuff which retains all its desirable organoleptic properties.